The present invention relates to high consistency disc refiners, and more particularly, to disc refiners which have confronting, counter-rotating discs defining two distinct refining zones therebetween.
In the field of rotating disc-type pulp refiners, a known refiner construction includes opposed, counter-rotating discs between which material, such as pulp, is introduced near the axis of rotation, and undergoes defibration as the material moves radially outwardly until discharged at the circumferential periphery of the discs. The defibration, or refining, of the fiber at high consistency produces considerable amounts of steam, which has two detrimental effects. First, the steam tends to carry the fiber radially outward to be discharged from between the discs, before refining has been completed. In other words, the steam generation tends to decrease the dwell time of the fiber in the refining zone between the discs. Secondly, the steam generated in the refining zone tends to push the discs axially apart, and therefore requires that the refining equipment produce a counter-thrust to maintain the gap between discs within a range that achieves defibration. The counter-thrust cannot be so great, however, to induce contact between the discs, which, due to the high rotation speeds, can damage the equipment and result in prolonged outages.
U.S. Pat. No. 4,283,016, issued Aug. 11, 1981 to Reinhall, discloses a method and apparatus for controlling the effect of centrifugal force on the pulp of a double disc defibrating apparatus. The grinding space includes a central portion, a first grinding zone defined between first and second rotating grinding discs and extending outwards from the central portion, and a second grinding zone extending angularly from the outer end of the first grinding zone and being defined between one of the rotatable grinding discs and a stationery grinding surface. Pulp stock to be ground is introduced into the central portion and accelerated through the first and second grinding zones by centrifugal force generated by the rotating discs. The angular second grinding zone serves to retard centrifugal force acting on the pulp in the second grinding zone to increase the dwell time of the pulp in the grinding space for achieving optimum refining efficiency.
The apparatus disclosed in the Reinhall patent is concerned primarily with retarding the flow of pulp in the refining zones, as a counter measure to the increase in centrifugal force associated with the increasing diameter of modern discs. Reinhall does not, therefore, address the effects on the refining process and apparatus, of the considerable amounts of steam generated in the refining zone.